Ectromelia virus (ECTV), the causative agent of mousepox (the smallpox of the mouse), is one of the few models available of a natural mouse pathogen that infects through the periphery and spreads through the lymphatic system to become systemic and cause disease. Experimental systems that lack this natural pairing will miss much of the complex interplay that has developed over a long period of co-evolution. The goal of the three programs in this project are to dissect the mechanisms that control a natural ECTV infection. The three projects in the Program will require genetically manipulated ECTV. In addition, the Core's pilot research project will require the deletion of two ECTV genes. The genomes of poxviruses can be manipulated by homologous recombination to either eliminate genes or to add foreign sequences and the procedures involved are relatively simple for experienced hands. While most of the techniques for the genetic manipulation of poxviruses were developed for vaccinia virus (VACV), we have successfully used homologous recombination to manipulate the ectromelia virus (ECTV) genome. In addition, Project 2 will need to create transgenic mice via genetic recombineering and to create BAG reporters. Members of the core have ample experience with both procedures. The Core's pilot project will be to study whether the ECTV immune response modifiers (IRMs) EVM043 and EVM145 play a role in virulence. These genes are known to interfere with the host Type 1 interferon response. Therefore, the research will complement experiments in Project 2 and is fully consistent with the overall goals of the Program.